The present invention generally relates to compositions containing α-sulfofatty acid ester and methods for making and using such compositions. More particularly, the present invention relates to compositions containing α-sulfofatty acid ester and hydrotrope, and methods for making and using the same.
Detergents have been used for many years to clean clothing and other materials. Detergents originally contained soap derived from animal fats. More recently, surfactants have been included in detergents to enhance their cleaning performance. Typical surfactants include anionics, nonionics, zwitterionics, ampholytics, cationics and those described in Surface Active Agents, Volumes I and II by Schwartz, Perry and Berch (New York, Interscience Publishers), Nonionic Surfactants, ed. by M. J. Schick (New York, M. Dekker, 1967), and in McCutcheon's Emulsifiers & Detergents (1989 Annual, M. C. Publishing Co.), the disclosures of which are incorporated herein by reference.
Anionic surfactants are a preferred type of surfactant for laundry detergents due to their improved cleaning performance. The cleaning performance of anionic surfactants can be limited, however, by water hardness. Calcium and/or magnesium ions in hard water interfere with some anionic surfactants, such as alkyl olefin sulfonates, alkyl sulfates, linear alkyl sulfonates, and linear alkyl benzene sulfonates. Recently, interest in α-sulfofatty acid esters (also referred to hereafter as “sulfofatty acids”) has increased due to the improved cleaning properties of these surfactants in hard water. While α-sulfofatty acid esters and other anionic surfactants have similar detergency in soft water, as water hardness increases α-sulfofatty acid esters exhibit better cleaning performance as compared with other anionic surfactants.
The use of α-sulfofatty acid esters has not been widely accepted, however, due to several disadvantages of such sulfofatty acids. In particular, α-sulfofatty acid esters tend to degrade to form di-salts during their manufacture. While mono-salts of α-sulfofatty acid esters have the desired surface active agent properties, di-salts have several undesirable properties that degrade the performance of the α-sulfofatty acid ester. For example, the Kraft point of a C16 methyl ester sulfonate (“MES”) di-salt is 65° C., as compared to 17° C. for the mono-salt form of C16 MES. (The Kraft point is the temperature at which the solubility of an ionic surfactant becomes equal to its critical micelle concentration; below the Kraft point, surfactants form precipitates instead of micelles.) Thus, the higher the Kraft point, the more di-salt precipitates in the composition. The resulting poor di-salt solubility in cool and even slightly hard water is a disadvantage in most applications. Thus, significant amounts of di-salt in otherwise high quality α-sulfofatty acid ester degrade the performance of that sulfofatty acid. The presence of large amounts of di-salt in α-sulfofatty acid ester, therefore, results in a poorer quality α-sulfofatty acid ester product, characterized by degraded performance and reduced application flexibility.
Di-salts also result from hydrolysis of α-sulfofatty acid ester during storage and in detergent formulations. In particular, mono-salts of α-sulfofatty acid ester hydrolyze in the presence of moisture and alkali-containing detergent components to form di-salts. For example, in formulations where MES is well mixed with high pH components under aqueous conditions, the MES will hydrolyze nearly completely to the di-salt form. High pH components include builders, such as silicates or carbonates, and bases, such as sodium hydroxide (NaOH). This chemical instability discourages the use of α-sulfofatty acid esters in many applications.
A related problem associated with α-sulfofatty acid ester-containing detergent compositions is pH drift. In concentrated solutions, the pH of the solution drifts towards the acidic (lower) range. Such pH drift interferes with other detergent components in the composition. To prevent pH drift, buffering or alkalizing agents are added to detergents. Buffering or alkalizing agents, such as caustic soda (NaOH), cause additional di-salt formation, however, which decreases the performance of the α-sulfofatty acid ester.
α-Sulfofatty acid esters also have limited solubility in concentrated solutions. For example, phase separation occurs in concentrated solutions of C16 or C18 α-sulfofatty acid esters if the sulfofatty acid ester is not adequately solubilized. To prevent phase separation, a hydrotrope is added to the detergent composition. (A hydrotrope is a compound that is soluble in aqueous solutions and that increases the aqueous solubility of organic compounds.) Common hydrotropes include urea, lower molecular weight alkanols, glycols, and ammonium, potassium or sodium salts of toluene, xylene or cumene or ethyl benzene sulfonates. The latter hydrotropes tend to be more expensive, so less expensive hydrotropes, such as urea ((NH2)2CO) or urea-alkanol mixtures, are frequently used as cost-effective substitutes. Greater quantities of these hydrotropes are required, however, to achieve the stabilizing effects of the more expensive hydrotropes.
A disadvantage of urea-based hydrotropes, however, is that contaminants in urea release unpleasant odors. In particular, urea often contains ammonium carbamate (NH4CO2NH2), which hydrolyzes to release ammonia. If ammonia is released during washing, it can offend the consumer, leading to decreased consumer satisfaction with the product. Urea itself also slowly hydrolyzes to release ammonia. If high levels of urea are present, such hydrolysis tends to increase the pH of the composition. Such high pH values are generally incompatible with some uses of α-sulfofatty acid esters and with other detergent components.
Thus, there is a need for a composition of α-sulfofatty acid ester and hydrotrope that stabilizes the α-sulfofatty acid ester and reduces additional di-salt formation. There is a further need for a hydrotrope that reduces pH drift and/or phase separation by α-sulfofatty acid esters. Surprisingly, the present invention satisfies these needs.